The 7801 is a very simple 8085A card with 1k of static RAM expandable to 4k and provision for 8k of ROM as 4x 2716 EPROMs.
The 8085A includes a simple serial port, but this card has no level shifters, so the levels are TTL.
Currently, this board has no ROM, but its memory map is the same as the SDK-85, so its monitor ROM may be suitable.
Pro-Log also developed a monitor ROM. I have a copy of this ROM, but it relies on having the 7303 Keyboard & Display Card, and i don’t have one. I have ordered some STD prototype cards with a view to creating a 7303 emulator.
Intel SDK-85 monitor ROM was suitable. It uses the 8085 serial terminal for user interaction – albeit at 110 baud (with 7 bits and no parity).
The SDK-85 monitor checks the state of the RX line on startup. If it is high, then it will use the SDK-85 keypad and seven segment displays (which the 7801 does not have). If it is low, then it uses the serial lines.
The 7801 does not include level shifters, so i removed a couple of ICs from a 7301 RS232 card, so i could use it to do the level shifting.
This was sufficient to show that the 7801 card is quite viable.
Rather than use such cumbersome level-shifting arrangement i thought a little TTL-RS232 converter off ebay would be a bit simpler. It did not go as smoothly as expected ….
It turns out that my test setup was somewhat fortunate in that the transmit line was not inverted. It seems that the serial transmission using the SDK-85 ROM is inverted with respect to the receive. This meant that i had to modify the 7801 to remove the TTL-TTL inversion at U10 (74LS240). This meant lifting pin 3 out of the socket and then connecting pin 17 to pin 3. I also had to add 5V to one of the unused pins on the connector socket, J1, so that i could power the level shifter. I used pin 8.